This invention is directed to electrochemical sensors having improved sensitivity, stability and durability. This invention is also directed to processes for the fabrication of such sensors and to methods for their use. In accordance with preferred embodiments of the invention, improved sensors are provided for the detection of species such as oxygen, sulfur, carbon and the like in harsh environments such as at high temperatures in corrosive environments, and in metallurgical and other melts.
The detection of elemental and molecular species is an important aspect of numerous industrial and other procedures. For example, the detection of oxygen, sulfur, carbon and other species in metallurgical melts, in flue gasses, in chemical reactors and in other high temperature or corrosive situations is critical to the metals, utilities, chemicals, vitreous products and other industries. Moreover, caustic, corrosive, oxidative and other harsh conditions and environments strain the ability of sensor manufacturers to prepare electrochemical sensing devices capable of surviving such conditions and environments while retaining the ability to perform the desired sensing duties in an accurate and reliable fashion and for reasonable periods of time.
One area presenting particularly stingent requirements for sensing devices is the field of metals purification and processing. It is commonly necessary to measure the oxygen, sulfur, carbon and other contents of molten metals such as iron and steel. To do so, it is known to immerse electro-chemical sensors in such melts. The electromotive force, EMF, generated by the sensor is then monitored and related to the activity or concentration of the atomic or molecular species of interest. Heretofore, electrochemical sensors for the testing of such melts have demonstrated severe shortcomings including short operating lifetimes (often only a few minutes), high failure rates, poor reproduceability, low sensitivity and other faults.
General considerations for the development of electro-chemical sensors, especially those useful in metallurgical melts, are discussed in Worrell, "Developing New Electrochemical Sensors", Proceedings of the Symposium on Metal-Slag-Gas Reactions and Processes, Electrochemical Society, Princeton, N.J., (May 1975), incorporated herein by reference. Thus, it has long been desired to provide electrochemical sensors, especially those suited for harsh environments, which exhibit improved sensitivity, long-term physical and electrochemical stability, and physical durability.
It has also long been desired to provide methods for the preparation of electrochemical sensors which are, at once, effective in obtaining improved sensor qualities while securing reduced manufacturing costs. Methods of sensing and measurement which provide more accurate, longer term, and more sensitive information have also been sought. Prior to the present invention, however, these long felt needs have not been satisfied. Improvements attempted by others have failed to attain the desired goals.
Electrochemical sensors adapted for the measurement of oxygen in liquids such as in metallurgical metals are known. See in this regard "Developing New Electrochemical Sensors", Worrell, supra; "Oxide Solid Electrolytes", Worrell, Topics in Applied Physics, Geller Ed., Springer (1977); and "The Measurement of Oxygen Chemical Potentials for the Calcium Fluoride Solid Electrolyte", Worrell et al., Journal of the Electrochemical Society, Vol. 126, No. 8, pp. 1360-1363 (1979). The foregoing references are incorporated herein by reference in order to describe more fully the electrochemical determination of elemental compositions employing solid electrolyte sensors. The foregoing publications disclose the employment of certain simple solid solutions such as those of zirconium oxide and thorium oxide in the electrochemical detection of oxygen.
Electrolytes suitable for the electrochemical measurement of sulfur or carbon under laboratory conditions have been disclosed. Calcium fluoride, for example, has been proposed for such use. See "The Measurement of Sulfur Chemical Potential Differences Using a Calcium Fluoride Solid Electrolyte," Worrell et al., Journal of the Electrochemical Society: Electrochemical Science and Technology, pp. 1717-1721, August 1980; and "Galvanic-Cell Investigation With a CaF.sub.2 Solid Electrolyte at Elevated Temperatures," Worrell, Solid State Ionics 3/4, pp. 559-563 (1981). Further attempts at the electrochemical measurement of sulfur have been reported. Thus, the employment of calcium sulfide-based electrolytes is reported in "Development of the High-Temperature Technology, IUPAC, pp. 503-509 (1969). In U.S. Pat. No. 4,428,770, in the names of the present inventors, novel sulfur and carbon sensors for metallurgical applications are disclosed. That patent together with each of the foregoing references are incorporated herein by reference. Other systems for measuring sulfur and carbon potentials are discussed in that patent.
For many years, oxygen sensors for metallurgical melts have been constructed from zirconia, ZrO.sub.2, partially stabilized with aliovalent materials such as calcium oxide, CaO.sub.2 or magnesia, MgO.sub.2. Thus, in one type of sensor a calcium oxide-stablized zirconia solid electrolyte pellet was welded into a quartz tube for insertion into melts. The tube was either open to a gas (such as the atmosphere) to provide a source of reference potential, or was provided with a solid reference electrode material such as metal-metal oxide mixture. EMF measurement and thermocouple means were also typically provided as appropriate. Such electrochemical sensors, while capable of some uses in metal melts, suffered from a lack of physical integrity leading to unreliable data, lack of reproducability, and failure after short periods of time when placed into metallurgical service.
In the 1970's efforts were made to develop improved solid electrolyte-based sensors, especially oxygen sensors, for metal melts. In this regard, it was hoped to overcome the physical instability and tendency toward thermal fracture of previous sensors through stabilization of the electrochemical material and improved physical processing methods for the sensor fabrication. The quartz tube was discarded and a tube formed from the electrolyte material itself employed. Partially stabilized zirconia, ZrO.sub.2 with about 3 wt % of MgO was blended together, formed into a tubular shape, compacted, and densified through sintering. The resulting, sintered electrolyte material comprises two phases, cubic and tetragonal, and exhibits improved mechanical strengths and resistance to thermal shock. The tube can be filled with a solid reference electrode material together with thermocouple and electrical lead means as desired.
The tube comprising a two-phased electrolyte can be used as an oxygen sensor in metallurgical melts and is widely used for this purpose today. While exhibiting utility for this purpose, the foregoing two-phase electrolyte tube devices still suffer from severe shortcomings. Such devices are extremely short-lived, being capable of use on the order of only a few minutes. While after special modification some tubes can last for as long as ten minutes before usefulness is lost, such lifetime is far less than is desired and the resulting data is of poor quality. Since the partially stabilized electrolyte in a two-phased mixture has, overall, an undesirably high electronic contribution to the conductivity, the device is incapable of accurately sensing oxygen concentrations below about 10 parts per million. Moreover, the electromotive force developed by such cells decreases substantially with time due to a progressive short-circuit of the cell. At the same time, such tubes are relatively expensive to manufacture.
The newest type of oxygen sensor proposed for use in metallurgical melts is the "needle sensor". See, in this regard, Janke, Solid State Ionics, Vol. 3/4, pp. 599-604 (1981). This sensing device is fabricated through an expensive sputtering technique whereby a molybdenum wire is typically coated with sputtered chromium-chromium oxide and then overcoated with sputtered stabilized zirconia. In has been found that such sensors are incapable of measuring low oxygen concentrations (less than about 20 ppm).
In the preparation of sensors such as oxygen sensors, which employ solid state electrolytes, it is known to be desirable to maximize the ionic conductivity of the electrolyte material and to minimize the electrical conductivity thereof. Sensing electrolytes having substantial electronic conductivity exhibit unstable results at low concentrations of oxygen when the electronic conductivity increases. It is also desired to maximize the intimacy of contact between the electrode material and that of the reference electrode material so as to minimize the equilibration time at the interface between the two materials. Of course, all of this must be obtained while maintaining acceptable physical integrity, mechanical strength and thermal shock resistance in the whole of the sensor.
The sensors in accordance with this invention solve the needs which exist for improved sensing devices and successfully address the requirements for improved sensors without suffering from the shortcomings exhibited by previous sensors.